Technical Field
The inventions relate to optical modulators and optical transmitters and methods for using optical modulators and optical transmitters.
Discussion of the Related Art
This section introduces aspects that may be helpful to facilitating a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Short-reach optical communications have used vertical-cavity surface-emitting lasers (VCSELs), which were directly modulated. For example, VCSELs may be used for optical communications networks inside data centers. For short reach applications, VCSELs can provide advantages of being inexpensive, small, easy to package, and usable with low drive
Typically, long-haul optical communication has used narrow line-width lasers and external optical modulators. In coherent optical communications, the external optical modulator may be an optical in-phase/quadrature-phase (I/Q) modulator with multiple optical phase modulators and/or amplitude modulators in a multi-arm interferometer. One such multi-arm interferometer is a nested Mach-Zehnder interferometer. Typically, for use as a digital data modulator, a Mach-Zehnder interferometer (MZIs) is constructed in lithium niobate, silicon, or group III-V semiconductor. Such MZIs typically produce low optical loss and low chirp of the optical carrier. Nevertheless, for phase modulation at high symbol rates, a MZI typically includes traveling wave modulation electrodes that are operates at large RF drive voltages (e.g., greater than 1 volt). Thus, in coherent optical communications, MZIs are typically large structures.
Typically, in long-haul communications, VCSELs are not used, because VCSELs usually have large line-widths, are often multimode, and may produce optical pulses with strong frequency chirp. Nevertheless, single-mode VCSELs have been used in coherent optical communication over distances of up to 960 kilometers when an optical receiver included a digital signal processor (DSP) configured to correct chromatic dispersion and polarization mode dispersion.